Faster speciation in the tropics?
This week's paper "Climatic zonation drives latitudinal variation in speciation mechanisms" is by Ken Kozak, a new member of my department, and John Wiens of Stony Brook University, published in Proceedings of the Royal Society. They used data on salamanders to test an old hypothesis to explain why there is so much species diversity in the tropics.
The hypothesis goes back to a 1967 paper by the eminent ecologist Dan Janzen, with the provocative title, "Why are mountain passes higher in the tropics?" They aren't, of course, but he suggested a pass at the same elevation may be more of a barrier in the tropics than in temperate regions. This seems odd, because it is presumably warmer at the top of a 2-km-high pass in the tropics than in Alaska, say.
Enter evolution. Tropical regions have less seasonal difference in temperature, at a given location, over the course of a year. Therefore, there is less selection for tolerance of a wide temperature range. Therefore, the top of a mountain pass may be colder than anything a tropical species has been exposed to over its recent evolutionary history. Therefore, tropical species can't usually cross even low mountain passes without dying. Therefore, two populations of the same species, on opposite sides of a pass -- maybe they were carried by migrating swallows, or crossed during a warm spell -- can't get together to interbreed. Without interbreeding, they diverge into separate species. In temperature regions, on the other hand, the temperature at the top of a mountain pass, in summer, is no colder than the temperature in the adjacent valleys, in winter. No problem. A potential date who is "geographically undesirable" for a tropical salamander with a narrow temperature range might be "just down the road" for a salamander whose ancestors had survived snowy winters.
Narrow climatic tolerance could lead to new species in at least two different ways, however. Under the "refuge/niche conservatism" hypothesis, species don't easily evolve tolerance to different climates. A species might move to higher elevations as climate warms, ending up split into isolated populations (which diverge into separate species) on different mountain tops. Alternatively, a species might spread up or down a mountain, evolving different, but still narrow, temperature tolerance as it goes. Limited temperature tolerance would prevent interbreeding up and down slope, leading to high- and low-elevation species. This is the niche divergence hypothesis.
Kozak and WIens used data on the distribution of salamanders in temperate and tropical America to test these hypotheses. They concentrated on pairs of "sister species", shown by DNA analysis to be adjacent twigs in the salamander family tree. They used distribution data and climate maps to determine the temperature tolerance of each species.
Consistent with Janzen's hypothesis, there was more temperature range overlap between sister species from temperate regions than from the tropics. Pairs of temperate species were usually separated by zones outside their temperature range, consistent with niche conservatism.
sister species in the temperate zone show much greater temperature overlap with each other than they do with their intervening absence locations, a pattern that supports the hypothesis that niche conservatism underlies their past and present isolation
This was also true of two of six species pairs in the tropics, but mostly sister species in the tropics were found in zones that differed in temperature at least as much as intervening zones did. They couldn't tell whether tropical species diverged while in contact with each other (parapatric speciation) or whether they were separated for a time, then came back into contact after they had so little in common they might as well be separate species.
Also this week
Has anyone noticed that I have never reviewed a paper in any of the 30+ journals in our library with "evolution" in the name of the journal? I've mostly stuck to general-interest journals for two reasons. First, this is supposed to be a general-interest blog. Second, I wanted to make the point that evolution is central to biology. I'm doing fine without my appendix, and junk DNA can be deleted without affecting fitness, but biology without evolution would be like chemistry without atoms. Even limiting myself to a few general-interest journals, I have several interesting papers to choose from each week. Here are some of this week's runners-up:
Human cooperation in social dilemmas: comparing the Snowdrift game with the Prisoner's Dilemma